Installed electrical power equipment is often subjected to diagnostic tests, such as partial discharge tests, requiring high alternating voltage at power frequency. Although power frequency is sometimes defined as any frequency in the range of 20 Hz to 300 Hz, in reality, most if not all terrestrial electrical equipment operate at either 50 Hz or 60 Hz. A diagnostic test conducted at a voltage having a steady amplitude and a frequency in the range of 50 Hz to 60 Hz is, therefore, most desirable, as it duplicates the day-to-day operating condition of the installed equipment. Thus, standard alternating voltage tests conducted in the factory by equipment manufacturers use power frequency sources that have steady alternating voltage at 50 Hz or 60 Hz.
Conventional power frequency voltage sources used in factories for high voltage tests consist in relatively heavy and bulky transformers that are not practical for transportation to the field where installed equipment needs to be tested. Fortunately, when the installed electrical equipment to be tested can be considered as an electrical capacitor, as is clearly the case for a power cable, it is possible to use a resonant test set to generate the desired test voltage. While conventional test sets are often significantly smaller and lighter than power frequency voltage sources of the same rating used in a factory, these conventional test sets are still generally heavy and bulky and often require a large, heavy-duty truck for transportation to the field. As a result, conventional test sets can be expensive to transport (due to, for example, fuel consumption and government regulations). In addition, it is often the case that the source voltage of these conventional test sets is generated directly from the engine of the trucks adding additional wear and tear to the engine resulting in higher maintenance costs for the trucks. The test voltages generated using this configuration can also be unstable due to changes in the operation of the engine and a response of the engine to the additional loading of the test sets and the cables under test.
In principle, a resonant test set consists of a relatively small alternating voltage source (with a voltage and power rating in the order of 1/25th to 1/50th of voltage and power rating of the test set) connected in series with an inductor (reactor) and the power cable to be tested. If L is the inductance of the reactor, C the capacitance of the cable, and f the frequency of the voltage source, resonance is said to be achieved when 2πfL=1/(2πfC). Under resonance conditions, the voltage across the test cable becomes a large multiple, Q, of that of the alternating voltage source. The multiple Q, also called the quality of the circuit, is usually in the range of 25-75. Thus, starting with a modest voltage magnitude of 1 kV, it is possible to generate approximately 25 to approximately 75 kV. As a length of the test cable increases, the capacitance of the cable increases proportionally. In order to achieve resonance, the inductance L and/or the frequency f have to be decreased accordingly.
There are generally two conventional types of resonant test sets: (a) sets with constant frequency but variable inductance; and (b) sets with constant inductance but variable frequency. An example of a conventional resonant test set with a constant frequency but variable inductance can include a variable inductance reactor having a high voltage winding, sometimes split over two coils built around one or two legs of a split magnetic core formed by two U-shaped magnetic paths facing each other across open air gaps. While one of these U-shaped cores is stationary, the other is connected to mechanical actuators which allow the gap to open or close. The entire assembly, including the mechanical actuators and the opposing U-shaped cores, is often housed in a relatively large metal tank, normally filled with insulating oil. The forces of electromagnetic origin on the faces of the core across air gaps tend to dictate mechanical and structural designs which result in heavy core assemblies. When testing objects of small capacitance, such as short cables, resonance while using these conventionally test sets cannot be achieved, and complex schemes often have to be incorporated in the test set to work around this limitation. In one of the schemes, the reactor is connected in parallel with the cable under test, while allowing the test set to function as an auto-transformer. Furthermore, with cables having large capacitances, the air gap of reactors used in conventional test sets is forced to assume large values (e.g., on the order of 15 cm or more). This, in turn, requires such conventional test sets to be much larger (and heavier) than desired for field testing.
An example of a conventional resonant test set with a variable frequency, but fixed reactor inductance, often includes a voltage source of variable frequency. In order to achieve resonance with cables spanning a few hundred meters to several kilometers, as encountered in commercial cable installations, it is often necessary to operate these conventional test sets within a relatively large range of frequencies, such as 20 Hz to over 300 Hz, resulting in an operation undesirably removed from normal operation at 50 or 60 Hz.